Certain compounds present in wood have a deleterious effect on sheet coefficient of friction. Resinous and fatty acids such as oleic, linoleic, linolenic, palmitic and/or stearic acid are liberated from wood species during the pulping process. Due to their relatively low surface energies, these materials reduce the sheet coefficient of friction. Although pulp processing and washing reduce the total amount of these compounds significantly, carry over into the papermaking process is inevitable. The presence of these compounds in the finished sheet has been determined to significantly reduce the coefficient of friction of the sheet. Furthermore, by increasing the amount of deinked pulp in newsprint fiber, a significant increase in the amount of fatty acids is present in the stock slurry because fatty acid surfactants are used in the deinking process.
The need for enhanced coefficient of friction properties of printing papers, such as newspaper, is based on handling requirements of the paper reels in the paper mill as well as functional performance of the substrate in the converting process to a newspaper for use in the general public. Low kinetic coefficient of friction papers in the mill experience reel telescoping issues, which makes it difficult to transport finished rolls of paper in the mill. Another issue is crepe wrinkles, where the sheet will slip upon itself after having been wound into a tight reel. As the sheet slips, ridges or wrinkles form in the paper web. Once wrinkled, the paper web is unsuitable for printing and converting into the end product. A low coefficient of friction sheet also exhibits slipperiness in the converting process by misregistering on the printing papers press and running ahead during printing papers press stops or slow downs. The run ahead in the pressroom can result in damage to the print plates as well as break out on the printing papers press.
Prior art methods have attempted to increase the coefficient of friction of printing papers. One approach involves the addition of additives such as talc, hydrous kaolin, calcined kaolin or precipitated silica to paper in order to increase the coefficient of friction. Talc and hydrous kaolin tend to reduce friction due to their platelet morphology. One problem associated with the use of these materials includes abrasion and wear on paper production equipment due to abrasiveness of these additives. Another disadvantage is the relatively high dosage requirements (1 to 4% of furnish) of the additive, which translates to higher costs of manufacture. Finally, increased process costs associated with the need for retention aids as well as the adverse effects created by the use of dispersing agents in these materials adds further costs of using these additives as friction enhancers. Synthetic precipitated silicas also require high usage rates to impact friction; however, they do not tend to have a deleterious effect on friction at higher loadings as does kaolin and talc.
Colloidal silica has been used for many years in the art to increase the coefficient of friction of paper and paperboard. The majority of the prior art discloses the coating of the surface of a paper sheet with colloidal silica to enhance sheet friction. U.S. Pat. No. 2,872,094 to Leptien; U.S. Pat. Nos. 4,452,723 and 4,418,111 to Carstens; U.S. Pat. No. 3,916,058 to Vossos; U.S. Pat. Nos. 3,901,987; 3,754,984 and 3,860,431 to Payne et al.; U.S. Pat. No. 5,466,493 to Mefford et al.; U.S. Pat. No. 5,569,318 to Jarrand; and Japanese Patent Application No. 05172989 to Yoshihiko et al. disclose the coating of a paper product with silica in order to enhance anti-skid properties.
Surface application is problematic in printing papers, especially in newsprint and light weight coated grades for several reasons. Paper produced with large proportions of mechanical fiber are generally produced on high speed paper machine and are low in strength and are difficult to size, which promotes some water resistance for improved printability. Size press or water box additions of colloidal silica are difficult because the low strength greatly increase the probability of sheet breaks. Although spraying the colloidal silica on the surface of the sheet is effective in the production of boxboard grade, the coating is not uniform and roughens the surface of the sheet, which makes the paper much less desirable for printing. In addition, colloidal silica sprayed on the sheet is accomplished after drying because the friction gain from the silica is diminished by skuffing action of paper passing over rollers in the process.
Another approach is to add the colloidal silica to paper pulp prior to converting to a paper product. U.S. Pat. No. 3,649,348 to Vossos; U.S. Pat. No. 2,643,048 to Wilson; and International Patent Application No. WO 89/06637 to Rushmere discloses the addition of silica to paper pulp; however, these references do not disclose adding silica to pulp at the point of addition in the invention herein.
U.S. Pat. No. 4,952,279 to Ikeda et al. discloses the addition of anionic silica to paper pulp. Ikeda et aL. does not explicitly recite where in the papermaking process the anionic silica is added to the paper pulp; however, it can be inferred from Ikeda et al. that the anionic silica is added at the headbox based on the concentration of the pulp in Example 3. U.S. Pat. No. 5,501,771 to Bourson discloses the addition of a retention system to paper pulp in the headbox. The retention system is composed of a cationic starch, a polyaluminum chloride, and anionic silica. One disadvantage of the prior art methods of Ikeda et al. and Bourson is that the concentration of the pulp is low, which means a higher concentration of anionic silica is necessary to impart anti-skid properties.
In light of the above it would be very desirable to have a method for enhancing the anti-skid properties of paper by using a low amount of colloidal silica. Such a method would be especially useful in the production of printing papers such as newspapers and light weight printing papers where surface treatment is either not feasible or not practical. During the production of newspapers and light weight printing papers, the newspapers and light weight printing papers are rapidly transported through the paper machine. Thus, treating the newspaper or light weight printing papers with an additive by coating or spraying techniques is not an efficient way to treat the newspaper or light weight printing papers with the additive. The present invention solves such a need in the art while providing surprising advantages.